1. Field of the Invention
The present invention relates to suspension bridges and more particularly to a streamlined box girder type suspension bridge for dispersing live loads applied to a deck.
2. Description of the Prior Art
Stiffening girder type suspension bridges are generally classified into several types, the primary types of which are a box girder, a plate girder, a truss girder and the like.
Of these stiffening girder types, the stream-lined box girder type has recently been most utilized for suspension bridges of a long span for the reasons as will be described hereinbelow.
One of the advantages of the box girder type is the reduction in wind drag on the deck to one third of that for the truss girder type. Furthermore, the box girder type has higher torsional stiffness, weight for weight, than any other types and therefore is convenient to deal with aerodynamic oscillations. Still further, the steel in the box section is capable of resisting stresses in several directions simultaneously, i.e., shear, torsion, lateral bending and the like, serving to save in weight of steel and consequently to reduce the cost of overall bridge construction.
Among the prior art box girder type bridges, a streamlined box girder type proposed by Gilbert Roberts (Canadian Pat. No. 678,259) is well known. A bridge embodying Roberts' streamlined box girder type suspension bridge is known as the Severn Bridge of England built in 1966.
Roberts' streamlined box girder suspension bridge attempts to achieve stability against the wind by streamlining the girder cross section, and is basically based on a design concept of approximating the girder cross section to a slender streamline shape by reducing the girder depth as much as possible on the premise that the structural strength of the box girder is maximally utilized.
In the conventional truss stiffened girder suspension bridges, the girder depth becomes inevitably large because the girder rigidity is increased by trusses, thus proving economically disadvantageous compared to the streamlined box girder type. On the other hand, this type bridge is advantageous in that the bridge is less likely to be affected by the wind despite its large girder depth as the wind easily passes through the truss intervals.
Compared to such truss stiffened girder structure, the streamlined box girder type has various advantages since the girder strength is fully utilized because of the inherent rigidity of the box girder structure as above discussed. These advantages are; radically reduced volume and weight of steel by the reduction in girder depth; stability against the wind arising from the streamlined girder cross section; and extremely easy maintenance of the bridge such as painting.
Despite such several advantages of the streamlined box girder suspension bridge as above enumerated, there still exists a grave problem in this type suspension bridge that the bridge structure becomes extremely sensitive against the live loads and the wind drag. This problem is grave enough to offset the economic advantages over the truss girder type such as the radical reduction in the plate thickness and volume of steel used. To cite an example, if the weight of girder per se is decreased for purposes as mentioned above, aeolian oscillation or buffeting of fine amplitude generates to the girder by the constant wind of even a comparatively low velocity in addition to the rapid increase of traffic vibrations caused by live loads. The light weight and box shaped cross section of this type of girders deteriorate the damping capacity of the girder in absorbing the vibrations through its structure compared to the truss girder. A comparatively large stress repeatedly acts on the hangers and girders, thereby causing damage to the hangers or girders.
As the streamlined box girder type suspension bridge has the girder cross section which is streamlined like wings, if a comparatively large wind which occurs only infrequently acts on the girder and causes dangerous oscillation, the aerodynamic stability of the bridge reaches the maximum and destroys the bridge. This is called the bending-torsional flutter where bending and torsion occur to the girder concurrently.
Thus, the streamlined box girder type suspension bridge contains two contradictory elements; one is the economical advantage of achieving the dynamic stability without increasing thickness of the girder cross section since the cross section is streamlined to reduce the wind resistance; another is that the reduced inherent resistance against external oscillation elements with the less girder weight as the depth and plate thickness are reduced in size as well as the likelihood of catastrophic vibration such as bending-torsional flutter caused by the slender wing-like streamlined shape of the girder cross section.